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Struct pollfd

Struct pollfd

The only difference between these two APIs is the interface provided to the caller. The select API requires that the application pass in an array of bits in which one bit is used to represent each descriptor number. When descriptor numbers are very large, it can overflow the 30KB allocated memory size, forcing multiple iterations of the process. This overhead can adversely affect performance.

The poll API allows the application to pass an array of structures rather than an array of bits. Because each pollfd structure can contain up to 8 bytes, the application only needs to pass one structure for each descriptor, even if descriptor numbers are very large.

Socket flow of events: Server that uses poll The following calls are used in the example: The socket API returns a socket descriptor, which represents an endpoint. The setsockopt API allows the application to reuse the local address when the server is restarted before the required wait time expires. The ioctl API sets the socket to be nonblocking. All of the sockets for the incoming connections are also nonblocking because they inherit that state from the listening socket.

After the socket descriptor is created, the bind API gets a unique name for the socket. The listen API call allows the server to accept incoming client connections. The poll API allows the process to wait for an event to occur and to wake up the process when the event occurs. The poll API might return one of the following values.

In this example, the timeout is set for 3 minutes in milliseconds. The poll API allows simultaneous connection with all descriptors in the queue on the listening socket.

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The send API echoes the data back to the client. The close API closes any open socket descriptors. Note: By using the examples, you agree to the terms of the Code license and disclaimer information.

Parent topic: Examples: Socket application designs.Each fd corresponds to one set. The fd field specifies the file descriptor being polled. The events field indicates the interested poll events on the file descriptor. If a pollfd array contains multiple pollfd entries with the same fd field, the "events" field in each pollfd entry is OR'ed. The revents field is not used.

Write returns the number of bytes written successfully or -1 when write fails.

poll() - Unix, Linux System Call

Upon return, if the ioctl call has failed, -1 is returned. A return value 0 means the ioctl is timed out. For each valid pollfd entry, the fd field indicates the file desciptor on which the polled events happened. The events field is the user specified poll events. The revents field contains the events occurred. The fd field of the pollfd structure indicates the file descriptor of interest.

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The events field contains 0. The revents field contains the currently polled events. The ioctl returns 0 if the file descriptor is not in the set. The pollfd structure pointed by pfd is not modified.

The ioctl returns a -1 if the call fails.

poll.h(7posix) [linux man page]

A signal was caught during the execution of the ioctl 2 function. The request argument requires a data transfer to or from a buffer pointed to by argbut arg points to an illegal address. See attributes 5 for a description of the following attributes:.

Applications will exhibit the best performance gain if the polled file descriptor list rarely change. When a file descriptor is closed but not removed from the monitored set, and is reused in subsequent open of a different device, you will be polling the device associated with the reused file descriptor. EFAULT The request argument requires a data transfer to or from a buffer pointed to by argbut arg points to an illegal address.By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service.

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There's a main loop that the server is running. In each loop I am checking all the fds at the array of struct pollfd until I find n sockets that have content for me where n is the number that poll returned. Also, poll is used without timeout -1 as argument. So if a new connection is requested at the listening socket I am inserting the socket and some client information into a active connections list I am keeping.

Should I define a small array of size 10 and if there are more clients re allocate memory for an array of size 20 or 50? Edit: I think I've found a solution with realloc and shifting with memmove each time a client disconnects to cover his socket at the fds array.

struct pollfd

Don't optimise until you know you have to; That's called premature optimisation, and it's a waste of time. If your profiler determines that your pollfd s are a significant bottleneck in your program, and your boss or professor says your program " isn't fast enough ", go for it. If the largest your list will get is 50, then just use a static array, set the fd members to -1don't bother shifting things around when you remove from the array The bottlenecks you're referring to will be insignificant for such a small number.

If you're trying to handle much larger numbers of clients as a worst case, you might be concerned about unused space trailing the array when handling smaller numbers The simplest algorithm I can think of doubles space on resizing.

Perhaps the most significant benefit to doubling on resize is the reduction in calls to realloc compared to linear resizing; Rather than accepting connections and calling realloc times, I'd prefer to accept connections and call realloc 10 or 11 times.

That seems like a fairly good fix. It increases cache locality, but at the cost of having to call memmove and realloc every time a client disconnects.

I wouldn't even consider "defragmenting" the array until my profiler indicates that poll is using up too much processor time. When inserting, I'd choose items from that stack if possible. If you must defragment, I suggest doing so based upon the size of that stack.

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Active 6 years, 10 months ago. Viewed 3k times. I am using poll for implementing the server side of a client-server model with multiplexing. What is the most efficient way to handle the array of poll? Thank you for your time Edit: I think I've found a solution with realloc and shifting with memmove each time a client disconnects to cover his socket at the fds array.

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I think we need more info. How many clients are you expecting to handle simultaneously, at the most?

struct pollfd

Is it a large number eg. What are their patterns? Do they open a connection, they send a command, recieve a response and then immediately disconnect? Not many, since it's an assignment. The client probably will complete his work in approximately 5 minutes. No, there is some more interaction, since it is a file synchronisation application.This file also defines the returned events flags, error returned events flags, device-type flags and input flags used in polling operations.

During a polling operation on both file descriptors and message queues, the ListPointer parameter points to a pollist structure, which can specify either file descriptors or pointers and message queues. The program must define the pollist structure in the following form:. The pollfd structure and the pollmsg structure in the pollist structure perform the following functions:. The format of the macro is:. The Declarator parameter is the name of the variable that is declared as having this type.

The pollfd and pollmsg structures defined in the poll. Purpose Defines the structures and flags used by the poll subroutine. Item Description pollfd [ f ] This structure defines an array of file descriptors or file pointers. The f variable specifies the number of elements in the array.

The m variable specifies the number of elements in the array.

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Item Description fd Specifies a valid file descriptor or file pointer to the poll subroutine. If the value of this field is negative, this element is skipped. This field specifies the events that have occurred. This can be any combination of the events requested by the events field. This field can also contain one of the following flags: POLLNVAL The value specified by the fd field or the msgid field is neither a valid file descriptor or pointer nor the identifier of an accessible message queue.

This structure defines an array of file descriptors or file pointers. This structure defines an array of message queue identifiers. Specifies a valid file descriptor or file pointer to the poll subroutine. Specifies a valid message queue ID to the poll subroutine. The events being tracked. Returned events.GitHub is home to over 40 million developers working together to host and review code, manage projects, and build software together.

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Sign up. Branch: master. Find file Copy path. Cannot retrieve contributors at this time. Raw Blame History. Poll provides a method for multiplexing input and output on multiple open file descriptors; in traditional BSD systems, that capability is provided by select. While the semantics of select differ from those of pollpoll can be readily emulated in terms of select -- which is how this function is implemented.

Unix Network Programming. Prentice-Hall, NOTES 1. Clapper All rights reserved. Exceptions include out of band data. You signed in with another tab or window. Reload to refresh your session. You signed out in another tab or window. This file, and the accompanying "poll. Poll provides a method for multiplexing input and output. While the semantics of select. Stevens, W. This software is released under the following BSD license, adapted from.

Copyright cBrian M. All rights reserved. Redistribution and use in source and binary forms, with or without. Private Functions. Map the poll structures into the file descriptor sets required. Map the poll timeout value into a select timeout.

struct pollfd

The possible. A NULL timeout structure tells select to wait indefinitely. Public Functions.The field fd contains a file descriptor for an open file. If this field is negative, then the corresponding events field is ignored and the revents field returns zero. This provides an easy way of ignoring a file descriptor for a single poll call: simply negate the fd field.

The field events is an input parameter, a bit mask specifying the events the application is interested in for the file descriptor fd.

If this field is specified as zero, then all events are ignored for fd and revents returns zero. The field revents is an output parameter, filled by the kernel with the events that actually occurred. These three bits are meaningless in the events field, and will be set in the revents field whenever the corresponding condition is true. If none of the events requested and no error has occurred for any of the file descriptors, then poll blocks until one of the events occurs.

The timeout argument specifies the minimum number of milliseconds that poll will block. This interval will be rounded up to the system clock granularity, and kernel scheduling delays mean that the blocking interval may overrun by a small amount. Specifying a negative value in timeout means an infinite timeout.

Specifying a timeout of zero causes poll to return immediately, even if no file descriptors are ready. If the sigmask argument is specified as NULL, then no signal mask manipulation is performed and thus ppoll differs from poll only in the precision of the timeout argument. Return Value. Site Search Library linux docs linux man pages page load time Toys world sunlight moon phase trace explorer.The array must contain at least one structure with a valid socket.

Upon return, this parameter receives the updated sockets with the revents status flags member set on each one that matches the status query criteria. This is not necessarily the number of sockets for which status is requested. Returns one of the following values. Value Description Zero No sockets were in the queried state before the timer expired. For each socket, a caller can request information on read or write status.

Error conditions are always returned, so information on them need not be requested. All sockets that do not meet these criteria and have no error condition will have the corresponding revents member set to 0. This flag is not supported by the Microsoft Winsock provider. This flag is not returned by the Microsoft Winsock provider. The number of elements not sockets in fdarray is indicated by nfds. This behavior is useful to an application which maintains a fixed fdarray allocation and will not compact the array to remove unused entries or to reallocate memory.

It is not necessary to clear revents for any element prior to calling WSAPoll. The timeout argument specifies how long the function is to wait before returning. A positive value contains the number of milliseconds to wait before returning.

Skip to main content. Exit focus mode. Return value Meaning Greater than zero The time, in milliseconds, to wait.

poll(3) - Linux man page

Zero Return immediately. Less than zero Wait indefinitely. Return value Returns one of the following values. A subsequent call to accept is guaranteed to complete without blocking.

A subsequent recv function request will return zero bytes. Winsock performs an alertable wait in this situation, which can be interrupted by an asynchronous procedure call APC scheduled on the same thread.

Issuing another blocking Winsock call inside an APC that interrupted an ongoing blocking Winsock call on the same thread will lead to undefined behavior, and must never be attempted by Winsock clients. Is this page helpful? Yes No.

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Any additional feedback? Skip Submit. An error occurred. An invalid parameter was passed. Priority data may be read without blocking.